Formula: Ca2(Mg,Fe)5(Si8O22)(OH,F)2
Reflected/transmitted colour: colourless, pale green to deep green
Pleochroism: tremolite non-pleochroic, ferro-actinolite strength depends on Fe-content; α pale yellow, yellowish green, β pale yellow-green, green, γ pale green, deep greenish blue
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.599-1.688, β = 1.610-1.697, γ = 1.620-1.705
Birefringence δ: 0.017-0.027
2Vα angle: 62-86°
Dispersion: r < v, weak
Optical orientation: β = y, γ Λ z = 11-28°
Optical axial plane: (010)
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